The present invention relates to a composite structure, in which a layer is fastened to a metal sheet the back of which is inaccessible. Such structure is used particularly for the installation of insulating coverings or claddings of facades in the building sector.
An insulating covering, for example, insulating coverings includes an insulating layer consisting of panels and a sealing lining, the assembly as a whole being fastened to a roof carrier element formed of ribbed metal sheets fastened to a framework. At the present time, use is made of self-drilling and -tapping countersunkhead screws of small diameter (diameter 3 mm to diameter 6 mm), of a length greater than the thickness of the insulating panels (10 to 30 mm more) and fastened by means of threads in the upper areas of the ribbed metal sheets. A metal distribution washer having a pierced and countersunk central cup and of small thickness and large diameter (approximately 70 mm) is interposed between each screw head and the upper face of the insulating panel. The sealing lining is subsequently adhesively bonded or welded to the upper face of the insulating panels and to the distribution washers which are theoretically in the upper plane of the insulating panels.
One version involves employing the fastenings, comprising a washer and screw, after the installation of a first sealing membrane. A second membrane subsequently covers the first membrane and the visible washers and screw heads. Another version involves employing the fastenings, comprising washers of elongate shape and the screws, along the edge of a width of a sealing lining. An adjacent width subsequently covers the first width and all the visible washers and screw heads.
The use of such screws is far from satisfactory. The screws are installed by means of an electric screwdriver of rapid rotational speed and high torque. It often happens, especially if the insulating panels are dense and scarcely compressible and if the metal sheet is thin, that the screw, once it has come into abutment on its washer, continues to rotate without being capable of penetrating. Consequently, the diameter of the hole in the metal sheet becomes equal to the outside diameter of the threads of the screw, and the fastening is no longer effective.
For the same reasons of high screwing speed and/or torque, the screw can be screwed too deeply into the ribbed metal sheet, the screw head being driven, together with the distribution washer which may even bend, into the insulating panel. In such case, the sealing lining is not applied onto a continuous and flat support, thus impairing its proper functioning. This phenomenon becomes more serious if the insulation used is highly compressible, for example panels of mineral wool. Moreover, an aesthetic defect appears on the underside of the roof. That is the screw deforms the area of ribbed metal sheet upwards, with the result that such sheet no longer is plane.
Furthermore, even if the screw is correctly installed, when the roof is completed the screw can be forced in by one or more thread flights in the region of the ribbed metal sheet under the effect of a load, for example when a person walks on the distribution washer and the screw head. In such case, the external threads of the screw enlarge the hole in the metal sheet, and the fastening is no longer effective.
To overcome these disadvantages and to guarantee a high reliability of the fastening, whatever the compressibility of the insulating panels, composite structures of the type comprising a layer fastened to a metal sheet by at least one fastening device have been proposed, Such device includes, on the one hand, a threaded assembly member consisting of a rod having at one end thereof a head for driving the rod in rotation, a threaded part at an opposite end and, between the threaded part and the head, a smooth part adjacent to the threaded part, and, on the other hand, a distribution element on which bears the head for driving such member in rotation. The length of the assembly member as far as the threaded part corresponds to the thickness of the layer and of the distribution element, if appropriate after a predetermined compression of the layer, plus the length of a deformation flange of the metal sheet.
U.S. Pat. No. 4,453,361 describes a structure of this type, in which the diameter of the smooth part of the assembly member is reduced substantially to the threadroot diameter of the threaded part.
This design has serious disadvantages:
the assembly member can oscillate freely in the hole in the metal sheet, and therefore it is impossible to fasten in this way either a vertical cladding or a flexible insulating layer surmounted by a sealing membrane which would quickly form folds;
moist air can infiltrate between the hole in the metal sheet and the smooth part and cause corrosion of the metal sheet. When rust has developed, the tearing resistance of the sheet falls sharply;
the assembly member is not safeguarded against inopportune unscrewing under the effect of vibrations.